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Date:      Wed, 26 Aug 2020 12:54:18 +0000 (UTC)
From:      Mateusz Guzik <mjg@FreeBSD.org>
To:        src-committers@freebsd.org, svn-src-all@freebsd.org, svn-src-head@freebsd.org
Subject:   svn commit: r364816 - head/sys/kern
Message-ID:  <202008261254.07QCsIbx027944@repo.freebsd.org>
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Author: mjg
Date: Wed Aug 26 12:54:18 2020
New Revision: 364816
URL: https://svnweb.freebsd.org/changeset/base/364816

Log:
  cache: relock on failure in cache_zap_locked_vnode
  
  This gets rid of bogus scheme of yielding in hopes the blocking thread will
  make progress.

Modified:
  head/sys/kern/vfs_cache.c

Modified: head/sys/kern/vfs_cache.c
==============================================================================
--- head/sys/kern/vfs_cache.c	Wed Aug 26 12:53:16 2020	(r364815)
+++ head/sys/kern/vfs_cache.c	Wed Aug 26 12:54:18 2020	(r364816)
@@ -1134,8 +1134,15 @@ cache_zap_locked_vnode(struct namecache *ncp, struct v
 		to_unlock = vlp2;
 	} else {
 		if (!mtx_trylock(vlp1)) {
-			error = EAGAIN;
-			goto out;
+			/*
+			 * TODO: Very wasteful but rare.
+			 */
+			mtx_unlock(pvlp);
+			mtx_lock(vlp1);
+			mtx_lock(vlp2);
+			mtx_unlock(vlp2);
+			mtx_unlock(vlp1);
+			return (EAGAIN);
 		}
 		to_unlock = vlp1;
 	}
@@ -1377,7 +1384,6 @@ negative_success:
 			error = cache_zap_locked_vnode(ncp, dvp);
 			if (__predict_false(error != 0)) {
 				zap_and_exit_bucket_fail2++;
-				cache_maybe_yield();
 				goto retry;
 			}
 			cache_free(ncp);
@@ -1465,7 +1471,6 @@ retry:
 	error = cache_zap_locked_bucket(ncp, cnp, hash, blp);
 	if (__predict_false(error != 0)) {
 		zap_and_exit_bucket_fail++;
-		cache_maybe_yield();
 		goto retry;
 	}
 	counter_u64_add(numposzaps, 1);
@@ -1584,7 +1589,6 @@ negative_success:
 			error = cache_zap_locked_vnode(ncp, dvp);
 			if (__predict_false(error != 0)) {
 				zap_and_exit_bucket_fail2++;
-				cache_maybe_yield();
 				goto retry;
 			}
 			cache_free(ncp);

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